Process for treating sulphate-containing waste water

ABSTRACT

A process for treating sulphate-containing waste water comprises introducing a feedstock comprising sulphate-containing waste water, into a pond. The sulphate-containing waste water is subjected to biological sulphate reduction in the pond, thereby to convert dissolved sulphate anions to dissolved sulphide anions. Treated waste water, containing the dissolved sulphide anions, is withdrawn from the pond.

THIS INVENTION relates to the treatment of water. More particularly, the invention relates to the treatment of waste water. Still more particularly, the invention relates to a process for treating sulphate-containing waste water.

According to the invention, there is provided a process for treating sulphate-containing waste water, which process comprises

introducing a feedstock comprising sulphate-containing waste water, into a pond;

subjecting the sulphate-containing waste water to biological sulphate reduction in the pond, thereby to convert dissolved sulphate anions to dissolved sulphide anions; and

withdrawing treated waste water, containing the dissolved sulphide anions, from the pond.

In order to effect the biological sulphate reduction in the pond, a metabolizable carbon source may also be introduced into the pond, with the metabolizable carbon source mixing with the waste water in the pond, and with metabolization thereof by organisms involved in the biological sulphate reduction occurring. The metabolizable carbon source may comprise an organic carbon source which exhibits a high chemical oxygen demand (‘COD’). The organic carbon source may be an effluent or waste product comprising organic material dissolved, suspended and/or carried in waste water, such as sewage, settled sewage, settled sewage solids, tannery waste water, brewery waste water, starch manufacture waste water, and paper pulp waste water. These waste waters provide metabolizable organic carbon and the necessary organisms for the biological sulphate reduction in the pond, although, naturally, the pond can, at least in principle, be seeded with suitable microorganisms for the biological sulphate reduction in the pond.

The pond may be a facultative pond. In particular, it may be a facultative pond having an anaerobic compartment, eg an anaerobic pit upflow digester. The pond may thus contain an oxygenated surface layer of water. The oxygenated surface layer of water may be provided by withdrawing treated waste water from the pond, subjecting it to polishing, in a polishing stage, for photosynthetic oxygen production, for reoxidation of residual sulphides, and for removal of nitrates and phosphates, and returning a portion of the algae-rich oxygenated polished waste water to the pond as the surface layer. Instead, or additionally, the pond may be fitted with surface aerators to provide, or to assist in providing, the oxygenated surface layer of water in the pond.

The polishing stage may comprises a high rate algal pond. The process may include passing polished waste water, containing algal biomass, from the high rate algal pond to a harvesting stage where algal biomass is harvested or separated from the polished waste water.

The sulphate-containing waste water may be a sulphate-containing metalliferous, ie metal-containing, waste water from which the metal component has been removed. The process may thus include pre-treating a sulphate-containing metalliferous waste water to remove dissolved metal cations therefrom, and using the resultant pre-treated sulphate-containing, metal-lean, waste water as the feedstock to the pond. However, at least in principle, it is possible, if desired, to treat such a metal- and sulphate-containing waste water directly in the facultative pond, in which case there will be settling of metal sulphide sludges in the anaerobic compartment of the pond.

The sulphate-containing metalliferous waste water may be mine effluent or waste water containing dissolved heavy metal cations, such as ferrous cations, and the dissolved sulphate anions. Instead, however, the waste water can be any other dissolved sulphate-containing waste water, such as zinc refinery waste water. These waste waters may also be acidic.

The pretreatment of the metalliferous waste water to remove the metal cations therefrom, may include adding a sulphide compound to the waste water in a mixing stage, with the sulphide compound reacting with the dissolved metal cations to form the corresponding insoluble metal sulphides, allowing the metal sulphides to precipitate from the waste water, and separating the precipitated metal sulphides from the waste water in a separation stage, eg in a settler.

The sulphide compound may be in liquid or gaseous form. For example, it may be hydrogen sulphide.

In one embodiment of the invention, at least part of the sulphide compound added to the mixing stage may be that obtained by withdrawing treated sulphide-containing waste water from the pond; stripping gaseous components, including hydrogen sulphide and carbon dioxide, therefrom in a stripping stage; returning the stripped waste water to the pond; mixing the gaseous hydrogen sulphide with the feedstock; and passing the carbon dioxide to the high rate algal pond as a nutrient. However, in another embodiment of the invention, at least part of the sulphide compound added to the mixing stage may instead or additionally be that obtained by withdrawing treated sulphide-containing waste water from the pond, and mixing it directly with the feedstock.

Instead, however, the sulphide compound added to the mixing stage may be obtained from an external source. The process may then include withdrawing sulphide-containing waste water from the pond, recovering sulphur therefrom as a product, and returning the sulphide-lean waste water to the pond.

The process may include removing a metal sulphide sludge from the separation stage, and feeding the sulphate-containing waste water from the separation stage directly to the pond. However, if desired, the process may include treating the waste water from the separation stage in an alkalinity generating stage, to increase the alkalinity thereof, and returning a portion of this waste water to the mixing stage, to increase the alkalinity of the waste water, eg to neutralize the feedstock waste water. The alkalinity generating stage may comprise a stress reactor, which may be in the form of a high rate algal pond. The process may then include feeding at least some of the algal biomass harvested in the harvesting stage, to this high rate algal pond. The process may also include withdrawing pre-treated algal-containing waste water from this high rate algal pond; separating algal biomass therefrom; returning this biomass to the high rate algal pond, and then feeding the sulphate-containing waste water to the facultative pond.

The invention will now be described in more detail with reference to the accompanying drawings and the examples given hereunder.

In the drawings,

FIG. 1 shows a simplified flow diagram of a process according to the invention for treating waste water; and

FIGS. 2 to 10 show, graphically, the results obtained in experiments which were conducted to simulate various of the stages of the process of FIG. 1.

Referring to FIG. 1, reference numeral 10 generally indicates a process for treating waste water, in accordance with the invention.

The process 10 includes a mixing stage 12, with a waste water feed line 14 leading into the stage 12.

A waste water line 16 leads from the mixing stage 12 to a settler 18, with a metal precipitate sludge withdrawal line 20 leading from the settler 18.

A waste water line 22 leads from the settler 18 to a high rate algal pond 24. A waste water line 26 leads from the algal pond 24 back to the mixing stage 12. A withdrawal line 28 leads from the pond 24 to a settler 30, with an algal biomass return line 32 leading from the bottom of the settler 30 back to the pond 24. A treated, ie sulphate-containing, waste water withdrawal line 36 leads from the settler 30, with a withdrawal line 34 optionally leading from the line 36.

The line 36 leads into a facultative pond, generally indicted by reference numeral 40. The facultative pond 40 comprises an anaerobic pit upflow digester 42, as well as a surface layer 44 of algae-rich oxygenated water.

A treated waste water line 46 leads from the pond 40 to a high rate algal pond 48, with an oxygenated water return line 50 leading from the pond 48 to the surface layer 44 of algae-rich oxygenated water in the pond 40.

A polished waste water line 52 leads from the pond 48 to a harvesting stage 54, with an algal biomass withdrawal line 56 leading from the harvesting stage 54 to the line 36. An algal biomass transfer line 58 leads from the harvesting stage 54 to the pond 24. A waste water withdrawal line 76 leads from the stage 54.

An organic carbon feed line 60 also leads into the pond 40.

A waste water line 62 leads from the pond 40 to a gas stripper 64 with a stripped water return line 66 leading from the stripper 64 back to the pond 40. A hydrogen sulphide line 68 leads from the stripper 64 to the mixing stage 12, while a carbon dioxide line 70 leads from the stripper 64 to the pond 48.

An optional waste water line 72 leads from the line 22 to the line 36. Thus, by using the line 72, the pond 24 and settler 30 are by-passed.

An optional line 74 leads from the line 62 to the mixing stage 12. Thus, when the line 74 is used, the stripper 64 can be dispensed with, if desired.

In use, acidic mine effluent or waste water, containing dissolved sulphate anions and dissolved metal cations, enters the mixing stage 12 along the flow line 14. In the stage 12, hydrogen sulphide, entering along the line 68 and/or the line 74, is mixed with the mine effluent waste water, thereby causing precipitation of the metal cations, as metal sulphides. Simultaneously, the pH of the waste water is increased by means of alkaline water entering along the flow line 26. For example, the waste water may be neutralized by means of the alkaline water.

The resultant mixed waste water passes along the flow line 16 into the settler 18, where the precipitated metal sulphides are separated out and withdrawn along the line 20. The sulphate-containing waste water passes along the flow line 22 to the high rate algal pond 24, which functions as a stress reactor. Additional algal biomass enters the pond 24 along the line 58.

Treated sulphate-containing waste water, containing also algal biomass, enters the settler 30 where algal biomass may be separated from the sulphate-containing waste water. The algal biomass is returned to the pond 24 along the line 32. Treated waste water is withdrawn from the settler 30 along the line 36. Some of this water can be withdrawn from the process along the line 34, if desired. However, at least part, eg all, of the sulphate-containing waste water from the settler 30 passes along the line 36 to the facultative pond 40.

A carbon source, eg sewage, also enters the facultative pond 40 along the flow line 60. Biological reduction of sulphates thus takes place in the pond 40, with the sulphates being converted biologically to sulphides. Sulphide-containing waste water is withdrawn along the line 62 to the stripping stage 64 where carbon dioxide and hydrogen sulphide are stripped therefrom, with the stripped liquor returning to the pond 40 along the line 66. The hydrogen sulphide passes along the line 68 to the mixing stage 12.

Treated sulphate-lean waste water is withdrawn from the pond 40 along the line 46 to the high rate algal pond 48 which acts to polish the water, ie photosynthetic oxygen production takes place in the pond 48, as does reoxidation of residual sulphides and removal of nitrates and phosphates. Algae-rich oxygenated water is returned to the pond 40 from the pond 48, along the line 50, to form a surface layer of algae-rich oxygenated water 44 in the facultative pond 40.

Carbon dioxide, recovered in the stripper 64, enters the pond 48 along the line 70. The carbon dioxide is a nutrient for the pond 48.

Polished sulphate-lean waste water is withdrawn from the pond 48 along the line 52, and passes to the harvesting stage 54. In the harvesting stage 54, algal biomass is harvested or separated from the waste water, and withdrawn along the line 56. Some algal biomass is returned to the pond 24 along the line 58, with the balance passing to the digester 42 along the line 56.

Treated metal-free and sulphate-free waste water is withdrawn from the harvesting stage 54 along the line 76.

In the process 10, the facultative pond 40 is thus used for treating high volumes of waste water, for sulphate reduction. Additionally, a complex ‘low-grade’ carbon waste source, eg sewage, is used in the process 10, so that the process 10 results in a net disposal of such waste carbon sources.

The process 10 thus employs sulphate reducing bacteria to treat acid mine waste water. Acid mine drainage pollution may be associated with large water volume flows and exceptionally long periods of time over which the drainage may require treatment. The process 10 thus provides a means of treating such acid mine drainage. The process 10 provides high rates of sulphate reduction and precipitation of metal sulphides, in a waste stabilization ponding process. Simultaneously there is co-disposal of organic waste, and generation of algal biomass as an independent carbon source for sulphate reducing bacterial production.

Advantages of using the process 10 include: the earth work pond 40 is substantially less costly than a steel reinforced reactor vessel; renewable algal biomass is reliably produced in large amounts in the process 10; and the pond 40 can support high levels of sulphate reducing bacterial activity.

A number of the stages of the process 10 were evaluated on laboratory or pilot plant scale.

Materials and Methods

Sulphates and sulphides were analyzed according to Standard Methods. Chemical oxygen demand (‘COD’) was analyzed using a Merck (trade mark) Spectroquant Kit. Chlorophyll was extracted into 100% acetone and quantified according to Lichtenhaler HK (1987) Chlorophylls and Carotenoids: pigments of photosynthetic biomembranes. Methods in Enzymology 148:350-371. Borosilicate glassware was used in the metal binding experiments and the metal solutions were prepared using distilled water. Metals were analyzed on a GBC909AA (trade mark) Atomic Absorption Spectrophotometer linked to a GBC (trade mark) integrator. Total organic carbon (‘TOC’) was analyzed using a Dohrmann (trade mark) 180 Total Organic Carbon Analyzer.

Metal binding and removal studies were performed in 250 ml Erhlemeyer flasks. Metal removal was controlled against pure metal solution precipitation at the same pH and at the various metal concentrations tested. Reported results represent the difference between control and experimental readings. A 10 hour settling period was allowed for removal of metal precipitate.

An 8 l upflow anaerobic reactor was fed media with the following composition (g.l⁻¹): NH₄Cl 0,5; K₂HPO₄ 1.0; MgSO₄.7H₂O 0.2; CaCl₂.2H₂O 0,1; FeSO₄.7H₂O 0,1; Na₂SO₄ 0.5. Dried Spirulina sp. was used as the organic substrate. The reactor was seeded with sludge from a methanogenic reactor treating raw sewage. Gas production was monitored as well as sulphate reduction between inlet and overflow ports.

The Spirulina sp. culture for the metal binding studies was isolated from a tannery waste stability pond (‘WSP’), grown and maintained in Zarouk's media (Zarouk C (1966) Contribution a l'Etude d'une Cyanophycee. Influence de Divers Facteurs Physiques et Chimiques sur las Croissance et al Photosynthese de Spirulina maxima, Thesis, University of Paris, France) at a constant temperature of 28° C. under cold white light with a light/dark cycle of 18:6 hours. Cells were harvested by filtration through a GF/C filter or a nylon mesh with a pore size of 50 microns.

Metal binding to Spirulina was measured in a culture harvested and resuspended in either Zarouk's media or water. The cultures were placed on a shaker at 60 rpm and metal of varying concentrations added. Samples were removed at time intervals and filtered through a 0.45 μm nylon membrane filter. The filter was digested with 200 μl concentrated HNO₃ and analyzed for metals as described above.

EXAMPLE 1

Tannery Effluent

The use of tannery effluent as a source of waste organic matter to provide the carbon and electron donor for the sulphate removal and sulphide production unit operations of the process was investigated. In addition to its high organic load tannery effluent also contains high levels of sulphate. Treatment plants are a rich source of adapted sulphate reducing bacterial (‘SRB’) making the system a useful research model for the study of SRB sulphide production processes.

A 1,5 m³ upflow anaerobic rector was used. The reactor was fed a stream of a tannery's mixed effluent drawn prior to treatment and mixed with a sulphate solution to produce a final concentration around 2000 mg.l⁻¹ SO₄ and a COD:SO₄<0.5. This mixture simulates the flow, in FIG. 1, of a metal sulphate solution entering directly the anaerobic compartment of the facultative pond 40. An hydraulic retention time (‘HRT’) of 3 days was sustained in the reactor for the 60 day duration of the experiment and no methane production was observed. FIG. 2 shows the start-up period of this study, the achievement of an optimum sulphate reduction in the reactor around 80% and a conversion rate of over 500 mg SO₄.1⁻¹ reactor volume.day⁻¹. FIG. 3 shows the blending of reactor overflow liquor with a synthetic mine water solution to effect metal sulphide precipitation (2000 mg.l⁻¹ SO₄; 200 mg.l⁻¹ Fe; final pH 7,2) and shows the removal of iron at levels about 8 times the anticipated stoichiometric removal rate.

Metal removal from two zinc refinery waste streams was also evaluated. FIG. 4 shows total metal removal from a waste water pond and from a slimes dam seepage collection on the site. Zinc recovery levels of 2640 mg.l⁻¹ and 438 mg.l⁻¹ were recorded in each case indicating the relatively large amounts of metals which can be removed using this approach.

EXAMPLE 2

Tannery Effluent

High rate pond technology for the treatment of tannery waste water was assessed in a facultative pond reactor followed by a high rate algal pond (‘HRAP’) supporting a near monoculture of the cyanobacterium Spirulina sp. The COD and sulphate/sulphide removal function of the system are given in Table 1.

TABLE 1 Performance of a facultative and high rate algal pond system treating tannery effluent showing sulphate reduction, sulphide oxidation and COD removal in the various stages of the process. Facultative pond Facultative Tannery anaerobic pond effluent compartment aerobic cap HRAP Sulphate as SO₄ ²⁻ 975 <1 989 809 Sulphide as Na₂S 285 1100 76.5 0.1 COD 2474 1216 1216 394

Table 1 shows 100% sulphate reduction in the anaerobic compartment, a 92% reoxidation of sulphide produced in the aerobic facultative zone, which caps the facultative pond, thereby controlling sulphide emissions, followed by 100% final oxidation in the HRAP. An oxypause was established at a depth of 0.5 m from the surface of the facultative pond. An 84% reduction in COD load was achieved across the system. The metal removal function of the system was monitored. The results are given in Table 2.

TABLE 2 Heavy metal removal from tannery effluent in the anaerobic compartment of a facultative pond feeding a high rate algal pond (‘HRAP’). Heavy metal adsorption by algal biomass in the HRAP is shown before and after the commissioning of the sulphide producing anaerobic unit operation in the flow path. Percentage total metal removal includes both biomass adsorption and metal sulphide precipitation. Biomass metal Biomass metal concentration before concentration after Total metal sulphide treatment sulphide treatment removed (mg · Kg⁻¹) (mg · Kg⁻¹ ) (%) Cadmium 5.96 <1 100 Chromium 25.8 <1 100 Cobalt 22.4 3.3 85 Iron 2012 795 60 Lead 219 2.3 99 Nickel 49.2 17.5 64 Zinc 218.5 22.5 90

Table 2 shows the influent metal load from the tanning operation adsorbed by the microalgal biomass in the HRAP before and after commissioning the anaerobic sulphate reducing stage of the operation. The metal reduction of final treated water to <1 mg.l⁻¹, achieved by the joint action of sulphide precipitation followed by biomass adsorption, provides a demonstration of the acid mine drainage (‘AMD’) polishing quality which may be anticipated in the process 10. A 90% COD removal was recorded and methane formed 10% of free gas produced by the system. At an operating pH 8.3 no free sulphide release was detected above the pond surface. FIG. 5 shows the results of an investigation of the sulphide tolerance of the Spirulina sp. culture which established in the HRAP described above. It was found, in a series of sealed flask studies (head space flushed with nitrogen gas to exclude oxygen), that after an initial adaptation, growth was sustained even at the daily addition to the culture of a 300 mg.l⁻¹ sodium sulphide. The complete oxidation of the sulphide load by microalgal activity provides an indication of the efficiency that can be expected from the capping of the facultative pond with the HRAP recycle liquor.

EXAMPLE 3

Algal Biomass

The use of Spirulina sp. biomass produced in the above system, ie in Example 2, was investigated as a carbon source for sulphate reduction. The objective was to utilize the rapidly biodegradable fraction for sulphide production and to retain in solution the more slowly degraded cell structural components to be utilized for the subsequent metal adsorption unit operation. Biomass was harvested from the HRAP described above, dried, resuspended in tap water and fed to an 8 l upflow anaerobic digester with an HRT of 1 day. After reactor stabilization on the algal feed, the slurry was fed in a synthetic sulphate effluent-containing 800 mg.l⁻¹ SO₄, and at algal concentrations of 4.8 and 10 g.l⁻¹ for 40 days in each case. FIG. 6 shows percentage sulphate removal over 20 days of stable operation for each feed rate with an average COD removal of approximately 25%, and a COD:SO₄ conversion efficiency for the utilized component of the 4 g feed of 1.4:1. No methane production was observed over the study period. Metal uptake capacity of the reactor overflow was measured for Cu, Zn and Fe over a concentration range of 500 to 3000 mg.l⁻¹. Results of this study are reported in Table 3.

TABLE 3 Metal removal capacity of an anaerobic sulphate reducing digester overflow liquor from a digester fed a slurry of Spirulina as sole carbon and electron donor source. Results are reported as percentage metal ion removed over a concentration range 500 to 3000 mg · l and standard deviations are reported in brackets. 500 1000 1500 2000 2500 3000 Metal mg · l⁻¹ mg · l⁻¹ mg · l⁻¹ mg · l⁻¹ mg · l⁻¹ mg · l⁻¹ Cu 79.2 (± 9.0) 77.4 (± 19.0) 66.3 (± 14) 46.4 (± 4) 50.6 (± 15.8) 51.1 (± 8.6) Zn 88.0 (± 13) 67.9 (± 6.2) 38.9 (± 16) 32.6 (± 16) 38.0 (± 23) 36.7 (± 19) Fe 100 51.0 (± 17) 41.3 (± 58) 61.0 (± 4.9) 57.1 (± 4) 60.3 (± 6.7)

Table 3 indicates that total metal removal levels are substantially higher than anticipated stoichiometric metal sulphide formation, which, in this study, would account for only about 5% of total metal ions removed.

The results indicate that the potential metal binding capacity of the SRB-digested algal biomass may be enhanced by passing through the digester. More complete digestion of algal biomass will, of course, occur at HRT longer than one day. Algal biomass production within the ponding process offers a degree of independence from external supply of carbon sources.

EXAMPLE 4

Alkalization

The process 10 involves the use of alkalinity produced by algal photosynthesis in the precipitation and recovery of heavy metals. In the process 10, the recycle of alkalinity from the HRAP 24, and possibly also from the anaerobic compartment of the pond 40, is used to achieve the successful precipitation of certain mixtures of heavy metals. Formation and precipitation of metal sulphide/carbonate/hydroxide mixtures plays an important role in reducing the major fraction of the metal load in the incoming AMD, prior to passing to the anaerobic compartment of the pond 40, and to HRAP 48.

The ability of Spirulina cultures to rapidly realkalize their aquatic environment, following acidification with AMD, is illustrated in FIG. 7 for a biomass loading of 3 ug.ml⁻¹ chlorophyll a. In practice a continuous loading regime would be established to ensure a stable and elevated operating pH. FIG. 8 shows the survival of a Spirulina culture (1 ug.ml⁻¹ Chl a) together with its ability to sustain alkalinity production under a continuous loading regime with 10% AMD addition/day⁻¹. (pH 2 and metal concentration in mg.l⁻¹ Fe=95; Cu=1.08; Zn=2.2; Pb=1.52). FIG. 9 shows the metal removal efficiency in this reactor for the different components of the partly treated AMD feed. The chlorophyll a level decline, compared to control cultures, indicates stress conditions in this reactor and low growth rates which have, in turn, been shown to lead to increased production of extracellular metal complexing compounds by this organism (see also Example 5 hereunder). Both sustained alkalinity production as an AMD neutralizing step, together with an additional metal sequestering function, play an important role in assisting a pre-treatment precipitation operation, such as a HRAP 24 in FIG. 1, where the bulk of the incoming metal load may be recovered prior to entering the anaerobic compartment of the facultative pond 40. Poor Spirulina production in this unit indicates the requirement for continuous biomass production under optimum conditions in a separate reactor, such as HRAP 48 in FIG. 1. This may also serve as the final polishing step for water leaving the system.

EXAMPLE 5

Metal Complexing Compounds

It is likely that the production of extracellular metal complexing compounds in the form of capsular polysaccharides peptidoglycan and other fragments released from living algal cells plays some role as metal bioadsorbants in the metal removal functions discussed hereinbefore (Example 4). Metal removal contributed by this fraction was estimated by measuring the metal binding capacity of the cell free fraction of a Spirulina culture. The extracellular fraction was measured as total organic carbon (‘TOC’). FIG. 10 shows percentage metal removal, at three pH values in the acid to neutral range, in the growth medium fraction from which the cells were removed by filtration through a GF/C glass fibre filter. The results show that the separable pre-digested extracellular fraction could account for between 20% to 40% of metal removal in the range evaluated and that this process operates best under acidic conditions. It was found that production of this fraction was linked to light stress with TOC yield increasing from 2.17 mg.l⁻¹ to 44.39 mg.l⁻¹ as light intensity was increased from 130 to 1000 μmoles.m⁻².sec⁻¹. Both high light and metal stresses may be manipulated in HRAP 24 to maximise production of the extracellular metal completing fraction.

Recovery of sulphide and alkaline/metal complexing streams in the process 10, which are then fed to a pre-treatment metal precipitation unit operation, offers the potential for the fine control of the selective precipitation of metal sulphide/carbonate/hydroxide complexes. This would enable the partial separation and refinement of incoming components suggesting the use of the process 10 in the remediation of a range of metal waste streams in addition to AMD.

In the process 10, an anaerobic compartment within a facultative pond is an effective sulphate reducing reactor for both COD reduction and the efficient removal of heavy metal contaminants. The HRAP 24, 48 not only serve as a final metal polishing step, but contribute to primary treatment of AMD by both neutralization and adsorption functions. The release of extracellular complexing compounds by microalgae and their role in metal binding also takes place. Algal biomass generated in the process provides a degree of independence from the supply of external carbon sources and is used in sulphide generation of SRB.

The process 10 thus involves an integration of various components of the ponding approach to AMD treatment, and has metal recovery and separation potential. 

What is claimed is:
 1. A process for treating sulphate-containing waste water, which process comprises introducing a feedstock comprising sulphate-containing waste water into a facultative pond having an anaerobic pit upflow digester and an oxygenated surface layer of water; mixing an organic carbon source which exhibits a chemical oxygen demand, as a metabolizable carbon source, with the waste water in the pond; subjecting the sulphate-containing waste water to a biological sulphate reduction in the pond, thereby to convert dissolved sulphate anions to dissolved sulphide anions, with metabolization of the carbon source by organisms involved in the biological sulphate reduction, occurring; and withdrawing treated waste water, containing the dissolved sulphide anions, from the pond.
 2. A process according to claim 1, wherein the organic carbon source is an effluent or waste product comprising organic material dissolved, suspended and/or carried in waste water, and is selected from the group consisting in sewage, settled sewage, settled sewage solids, tannery waste water, brewery waste water, starch manufacture waste water, and paper pulp waste water.
 3. A process according to claim 1, wherein the oxygenated surface layer of water is provided by withdrawing treated waste water from the pond, subjecting it to polishing, in a polishing stage, for photosynthetic oxygen production, for reoxidation of residual sulphides, and for removal of nitrates and phosphates, to produce algae-containing oxygenated polished waste water and returning a portion of the algae-containing oxygenated polished waste water to the pond as the surface layer.
 4. A process according to claim 3, wherein the polishing stage comprises an algal pond.
 5. A process according to claim 4, which includes passing polished waste water, containing algal biomass, from the algal pond to a harvesting stage where algal biomass is harvested or separated from the polished waste water.
 6. A process according to claim 4, wherein the sulphate-containing waste water is a sulphate-containing metalliferous waste water from which the metal component has been removed.
 7. A process according to claim 6, which includes pre-treating a sulphate-containing metalliferous waste water to remove dissolved metal cations therefrom, and using the resultant pre-treated sulphate-containing waste water as the feedstock to the pond.
 8. A process according to claim 7, wherein the sulphate-containing metalliferous waste water is mine effluent or waste water containing dissolved heavy metal cations, and the dissolved sulphate anions.
 9. A process according to claim 7, wherein the pretreatment of the metalliferous waste water to remove the metal cations therefrom, includes adding a sulphide compound to the waste water in a mixing stage, with the sulphide compound reacting with the dissolved metal cations to form the corresponding insoluble metal sulphides, allowing the metal sulphides to precipitate from the waste water, and separating the precipitated metal sulphides from the waste water in a separation stage.
 10. A process according to claim 8, wherein the sulphide compound is hydrogen sulphide.
 11. A process according to claim 9, wherein at least part of the sulphide compound added to the mixing stage is that obtained by withdrawing treated sulphide-containing waste water from the pond; stripping gaseous components, including hydrogen sulphide and carbon dioxide, therefrom in a stripping stage; returning the stripped waste water to the pond; mixing the gaseous hydrogen sulphide with the feedstock; and passing the carbon dioxide to the algal pond as a nutrient.
 12. A process according to claim 9, wherein at least part of the sulphide compound added to the mixing stage is that obtained by withdrawing treated sulphide-containing waste water from the pond, and mixing it directly with the feedstock.
 13. A process according to claim 9, wherein the sulphide compound added to the mixing stage is obtained from an external source, with the process including withdrawing sulphide-containing waste water from the pond, recovering sulphur therefrom as a product, and returning the sulphide-lean waste water to the pond.
 14. A process according to claim 9, which includes removing a metal sulphide sludge from the separation stage, and feeding the sulphate-containing waste water from the separation stage directly to the pond.
 15. A process according to claim 9, which includes treating the waste water from the separation stage in an alkalinity generating stage comprising an algal pond, to increase the alkalinity thereof, and returning a portion of this waste water to the mixing stage, to increase the alkalinity of the waste water.
 16. A process according to claim 1, wherein the pond is fitted with surface aerators to provide, or to assist in providing, the oxygenated surface layer of water in the pond.
 17. A process for treating sulphate-containing waste water, which process comprises introducing a feedstock comprising sulphate-containing waste water into a facultative pond having an anaerobic compartment and an oxygenated surface layer of water; mixing a metabolizable carbon source with the waste water in the pond; subjecting the sulphate-containing waste water to biological sulphate reduction in the pond, thereby to convert dissolved sulphate anions to dissolved sulphide anions, with metabolization of the carbon source by organisms involved in the biological sulphate reduction, occurring; withdrawing treated waste water, containing the dissolved sulphide anions, from the pond; subjecting the withdrawn treated waste water to polishing, in a polishing stage, for photosynthetic oxygen production, for reoxidation of residual sulphides, and for removal of nitrates and phosphates, to produce algae-containing oxygenated polished waste water; and returning a portion of the algae-containing oxygenated polished waste water to the facultative pond as an oxygenated surface layer of water therein.
 18. A process for treating sulphate-containing waste water, which process comprises introducing a feedstock comprising sulphate-containing waste water into a facultative pond having an anaerobic compartment, an oxygenated surface layer of water, and surface aerators for providing, or assisting in providing, the oxygenated surface layer of water in the pond; mixing a metabolizable carbon source with the waste water in the pond; subjecting the sulphate-containing waste water to biological sulphate reduction in the pond, thereby to convert dissolved sulphate anions to dissolved sulphide anions, with metabolization of the carbon source by organisms involved in the biological sulphate reduction, occurring; and withdrawing treated waste water, containing the dissolved sulphide anions, from the pond. 